The 135 is wat replaced the 35, the 35 is what replaced the TO 35. There isn't a ton of difference to them but they are each a different tractor. The 135 is the most up to date technologically speaking but the 35 is a good machine too.
If I was to rate each of them I would say the TO 35 is good, the 35 is better with the 135 as best.
The 135 has a two stage clutch, live PTO, some with power steering, some are gas some are deisel, some have ROPS systems in place some don't. The 135 is still easy to get parts for, all three engines put in the 135 are great engines.
That being said, if you're in the market for a 40 hp range tractor the Allis Chalmers D14 and D15 or the 160 are also good reliable machine, as well as previously mentioned the JD 2020 or 2010, International 606's, Massey Ferguson 135, 150, 231, 235, 245, Ford 3000, 4000, (or 3400 deisel).
Basically any of the more popular models with from the bigger manufacturers (of the time) such as AC, MF, JD, Case, IH, Ford, or Oliver that is out of the late 60's-early 80's will be a good reliable machine.